Control systems are known for controlling engine air/fuel ratio in response to exhaust gas oxygen sensors positioned upstream and downstream of a catalytic converter. In such systems it is known to test efficiency of the catalytic converter by computing an amplitude ratio of downstream sensor to upstream sensor output and comparing the computed amplitude ratio to a reference ratio during a predetermined time period. In another known approach, a ratio of downstream to upstream switching ratios is compared to a reference ratio during a predetermined time period. For both approaches, degradation in converter efficiency is indicated when the calculated ratio falls below the reference ratio.
The inventors herein have recognized numerous problems with the approaches described above. For example, there may be an undesired variability in test results and, consequently, erroneous failure indications. Causes for such variability include: poor exhaust gas mixing; variations in vehicular operating conditions from one test period to another test period; and clipping of the sensor output during transient air/fuel operation.